Image forming apparatus having improved resolution switching capabilities

ABSTRACT

An image forming apparatus comprises image information which outputs a sync signal and means generated image information synchronously with the sync signal; image forming means form an image on the basis of the image information at either one of a plurality of resolutions and which generates a sync request signal to request a transmission of the sync signal; and a resolution changeover means. When the image forming unit received the resolution change over request signal before the sync request signal is generated, the image forming unit executes the switching operation of the resolution, and if the image forming unit received the resolution changeover request signal after the sync request signal had been generated, the image forming unit does not execute the switching operation of the resolution.

This application is a continuation of application Ser. No. 07/578,443filed Sep. 7, 1990, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus such as alaser printer to form an image by pixels and, more particularly, to animage forming apparatus in which a plurality of resolutions areselectively switched and an image can be printed.

2. Related Background Art

Hitherto, for instance, a laser beam printer has been known as such akind of apparatus. A printer engine unit of the apparatus has aconstruction as shown in FIG. 1.

In FIG. 1, reference numeral 1 denotes a photo sensitive drum as anelectrostatic latent image carrier; 2 indicates a semiconductor laser; 3a developing device for depositing a toner to an electrostatic latentimage and for visualizing; 4 a rotary polygon mirror to scan a beamwhich is emitted from the semiconductor laser 2 onto the photo sensitivedrum 1; 5 a charger; 6 a transfer charger to copy transfer a toner imageonto a print paper 10; 7 a fixing device to fix the toner which wastransferred onto the print paper 10; and 8 resist rollers for correctingthe oblique movement of the paper which was fed by a paper feed roller 9by once abutting the paper to the rollers and for adjusting the frontedge of the page and the image print start timing of the laser.Reference numeral 12 denotes a cleaner to clean the surface of the photosensitive drum 1 and 14 indicates a scanner motor to drive the polygonmirror 4.

To switch a resolution in the printer having such a construction, forinstance, it is necessary to switch a rotational speed of the scannermotor 14 and to switch a period of image clock corresponding to one dotof the turn-on/off of the semiconductor laser 2. In the conventionalswitching method, a switch or the like provided for the printer isdetected at the time of the turn-on of the power source and theresolution is switched when the printer is made operative.

Therefore, the resolution cannot be switched in a state of the power ON.

On the other hand, FIG. 2 shows a system in which the resolution isswitched by a resolution changeover request command which is sent from acontroller 30 as a generation source of image information. In FIG. 2,reference numeral 20 denotes a command which is sent from the controller30 to a printer engine 34 and 21 indicates a status which is contrarilysent from the printer engine 34 to the controller 30. The command andstatus signals are communicated asynchronously with an image signal.Reference numeral 23 denotes a timing signal of the image signal. Ahorizontal sync signal (well-known beam detection signal) and a verticalsync request signal are sent from the printer engine 34 to thecontroller 30. The vertical sync signal is sent from the controller 30to the printer engine 34. When the controller 30 receives the verticalsync request signal from the printer engine 34, the controller 30 sendsa vertical sync signal to the printer engine 34 and, thereafter, sendsimage data 26 of one line synchronously with a horizontal sync signalfrom the printer engine 34 and with an image clock 27.

On the basis of the image data 26, the semiconductor laser 2 in anoptical system unit 33 of the printer engine 34 is turned on and off. Alaser beam emitted from the laser 2 scans on the photo sensitive drum 1by the polygon mirror, thereby forming an image.

In addition to the optical system unit 33, the printer engine 34 alsoincludes: a scanner motor driver 32 to drive the scanner motor; and aCPU 31 to drive and control the photo sensitive drum 1, scanner motor14, and the like.

When the controller 30 outputs a resolution changeover request commandas a command 20, the printer engine 34 finishes the delivery of thepaper which is at present being printed and changes a rotational speedof the scanner motor 14 to a predetermined value and, thereafter,restarts the paper feeding operation.

However, in the conventional example, in the case of switching theresolution by the detection of the switch at the time of the turn-on ofthe power source, the resolution corresponding to the switch is setsimultaneously with the start of the operation of the printer.Therefore, for instance, the resolution cannot be switched every pageand the like. The requests from the controller 30 cannot be sufficientlysatisfied.

On the other hand, even in a printer which can switch the resolution bythe command 20 from the controller 30, after the paper 10 which is atpresent being printed had been once delivered and the rotational speedof the scanner was changed, the paper feeding operation is started.Thus, a throughput largely deteriorates due to the switching of theresolution.

Further, the above conventional technique has a drawback such that whenthe print paper 10 has once been fed, the resolution for the paper 10cannot be switched, so that it is difficult for the controller 30 toprocess such a situation.

In the above image forming apparatus, a predetermined amount of laserbeam is emitted in order to obtain the foregoing beam detection signal(hereinafter, referred to as a "BD signal") as a horizontal sync signalserving as a reference of the image forming position in the mainscanning direction.

However, due to the light emission to detect the BD signal, forinstance, an image of about two lines is formed as an electrostaticlatent image into an image effective area on the photo sensitive drum 1.Such an unnecessary electrostatic latent image which is not an inherentcopy transfer image is also developed as a toner image and the toner istransferred from the drum 1 to the transfer charger 6.

That is, hitherto, when the rotational speed of the scanner motor 14 ischanged in order to switch the resolution, an unnecessary toner image iscertainly formed when the BD signal is detected.

In the case of the system such that the transfer charger 6 is used upontransfer, the toner drops to a region around the transfer charger 6 andbecomes a cause of a failure of the transfer charger 6 or adeterioration of the performance thereof.

In the system in which a transfer roller (not shown) is used in place ofthe transfer charger 6, the toner image is directly transferred to thetransfer roller.

Such a fouling of the toner onto the transfer roller causes aninconvenience such that even if an amount of toner which is depositedonto the transfer roller is very small, what is called a "back surfacedirt" of a recording medium occurs by the elapse of time.

On the other hand, in a recording apparatus which can print images toboth surfaces of a paper, such a "back surface dirt" becomes a dirt ofthe recording surface and causes an inconvenience such that therecording picture quality is remarkably deteriorated.

SUMMARY OF THE INVENTION

It is an object of the invention to solve the foregoing technicalsubjects.

Another object of the invention is to solve various problems occurringby the switching of the resolution.

Still another object of the invention is to provide an image formingapparatus which can minimize a reduction in throughput occurring by theswitching of the resolution.

According to a preferred embodiment of the invention, a light beam isscanned by using a polygon mirror and a rotational speed of a scannermotor is switched when the resolution is switched.

It is, therefore, further another object of the invention to solvevarious problems which occur by the switching of the rotational speed ofthe scanner motor.

The above and other objects and features of the present invention willbecome apparent from the following detailed description and the appendedclaims with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a construction of a printer engine sectionin each of the conventional example and the first and secondembodiments;

FIG. 2 is a diagram showing a construction of an image recordingapparatus in each of the conventional example and the first and secondembodiments;

FIG. 3 is a control flowchart of the printer engine section in the firstembodiment;

FIG. 4 is a control flowchart of the printer engine section in thesecond embodiment;

FIG. 5 is a diagram showing a construction of a printer engine sectionin the third embodiment;

FIG. 6 is a control flowchart of the printer engine section in the thirdembodiment;

FIG. 7 is a block diagram showing the fourth embodiment according to theinvention;

FIG. 8 is a diagram showing the details of the relation between acontroller unit and a scanner unit in the fourth embodiment;

FIG. 9 is an operation timing chart in the fourth embodiment;

FIG. 10 is a diagram showing the relations among the respective sectionsin a recording unit in a cleaning sequence for a transfer output in thefourth embodiment;

FIG. 11 is a block diagram showing the fifth embodiment according to theinvention;

FIG. 12 is an operation timing chart in the fifth embodiment;

FIG. 13 is a block diagram showing an electrical arrangement of thesixth embodiment according to the invention;

FIG. 14 is a diagram showing a construction of a printer engine sectionof the sixth embodiment;

FIGS. 15 and 16 are control flowcharts of the sixth embodiment; and

FIG. 17 is an operation timing chart in the sixth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments according to the present invention will bedescribed in detail hereinbelow with reference to the drawings.

(First embodiment)

FIG. 3 is a control flowchart of the printer engine 34 showing the firstembodiment according to the invention.

A construction of a whole apparatus is similar to the constructionsshown in FIGS. 1 and 2 which have already been described with respect tothe conventional example and, therefore, its description is omittedhere.

An algorithm to switch a resolution when the first page is printed willnow be described.

First, when a power source of a printer is turned on, the selfinitialization processing is executed in step S1 in FIG. 3. Aftercompletion of the processing, step S2 follows. In step S2, a check ismade to see if a print request signal has been output from thecontroller 30 or not. If NO, step S3 follows and a check is made to seeif a resolution changeover request signal has been output or not. If YESin step S3, step S4 follows and a processing to switch to the requestedresolution, for instance, a processing to set the rotational speed ofthe scanner motor 14 is performed. In step S5, a check is made to see ifthe changeover processing has been completed or not. If YES in step S5,the processing routine is returned to step S2 and the apparatus waitsfor the reception of a print request signal.

If the resolution changeover signal is received after the print requestsignal was received, the processing routine advances from step S2 tostep S6 and the paper feeding operation by the paper feed roller 9 isstarted. After that, in steps S7 and S8, checks are always made to seeif a paper has reached the resist rollers 8 or not and to see if theresolution changeover signal has been transmitted or not. If theresolution changeover signal is not received for a period of time whenthe paper reaches the resist rollers 8, the processing routine advancesto step S12. In step S12, in a manner similar to the ordinary print, avertical sync request signal is output to the controller and the imageprint processing is started.

However, if the resolution changeover signal was received before thepaper reaches the resist rollers 8, step S9 follows and the resolutionchangeover processing is soon started. In step S10, a check is made tosee if the changeover processing has been completed or not. If YES, stepS11 follows and a check is made to see if the paper has reached theresist rollers 8 or not. If NO in step S11, the apparatus waits untilthe paper reaches the resist rollers 8. After that, if the paper hasarrived at the resist rollers 8, the processing routine advances to stepS12 and the foregoing print processing is executed. However, if theresolution changeover processing is not finished even after the paperhas reached the resist rollers 8, the paper is held and stopped at theposition of the resist rollers 8 until the changeover processing iscompleted. That is, in the first embodiment, if the changeoverprocessing is not finished, the paper is held at the position of theresist rollers 8, thereby controlling so as not to output the verticalsync request signal.

(Second embodiment)

The second embodiment according to the invention will now be describedhereinbelow in accordance with a control flowchart shown in FIG. 4.

A construction of a whole apparatus is similar to the constructionsshown in FIGS. 1 and 2 which have already been described in theconventional example and, therefore, its description is omitted here.The same processings as those in the first embodiment are designated bythe same step numbers.

An algorithm in the case where the resolution changeover signal wasreceived during the continuous printing operation will now be shownhere.

Similarly to the first embodiment, the paper feeding operation isstarted during the continuous printing operation (step S6). Checks aremade to see if the paper has reached the resist rollers 8 or not and tosee if the resolution changeover request signal has been sent or not(steps S7 and S8). If the resolution changeover request signal is notreceived until the paper reaches the resist rollers 8, the vertical syncrequest signal is output (step S12) in a manner similar to the ordinarycase. However, if the resolution changeover request signal was receivedbefore the paper reaches the resist rollers 8, a check is made to see ifan image has been printed at this time point or not (step S14). If YESin step S14, the apparatus waits until the completion of the image printprocessing.

After that, the resolution changeover processing is started. Then,checks are made to see if the changeover processing has been completedor not and to see if the paper has reached the resist rollers 8 or not(steps S10, S11). If the changeover processing had been completed at thetime point when the paper has reached the resist rollers 8, the verticalsync request signal is output at the ordinary timing. On the contrary,if the changeover processing is not finished at the time point when thepaper has reached the resist rollers 8, the print paper is held at theposition of the resist rollers 8 until the changeover processing isfinished. After completion of the changeover processing, the verticalsync request signal is output.

(Third embodiment)

FIG. 5 is a diagram showing a structure of a printer engine section inthe third embodiment according to the invention. In the thirdembodiment, the resist rollers 8 used in the foregoing embodiments andthe conventional example do not exist and only the paper feed roller 9exists between the paper cassette 10 and the photo sensitive drum 1.

An algorithm of the resolution changeover in such a paper feeding systemwill now be described with reference to a flowchart of FIG. 6. In thethird embodiment, the same processings as those in the first and secondembodiments are designated by the same step numbers.

First, if the resolution changeover request signal is received in astate in which a print request signal is not received from thecontroller (not shown), that is, in a standby state, the processingnecessary to switch the resolution is soon started (steps S3, S4). Aftercompletion of the changeover processing, a check is again made to see ifthe print request signal has been received or not (step S2).

If the resolution changeover request signal has been received after theprint request signal had been output from the controller, the resolutionchangeover processing is first started (step S9). The paper feedingoperation is stopped until all of the processing which are necessary forthe resolution changeover are finished. After completion of thechangeover processing (step S10), an image request signal is output tothe controller (step S15) and the paper feeding operation is started(step S6).

On the other hand, if the resolution changeover request signal is notreceived when the print request signal was sent, the image requestsignal is immediately output and the paper is fed.

According to the first to third embodiments, the apparatus is set suchthat the resolution changeover request can be accepted until thevertical sync timing for the page to be printed. The resolutionchangeover processing is executed at a timing other than the period oftime of the image printing operation. Further, if the changeoverprocessing is not completed at the time point when the vertical synctiming has come or if an image of the preceding page is being printed,by providing a function to stop the paper conveyance at that position,the resolution change-over can be flexibly performed and the decrease inthroughput upon switching of the resolution can be minimized.

(Fourth embodiment)

The fourth embodiment according to the invention will now be describedhereinbelow with reference to FIGS. 7 to 10.

FIG. 7 is a diagram showing a structural conceptual construction of arecording section of a laser beam printer (hereinafter, referred to asan "LBP") based on the electrophotographic method in which the laserbeam in the fourth embodiment is used as exposing means according to theinvention.

In FIG. 7, reference numeral 100 denotes a controller unit; 200indicates a high voltage power source; 300 a laser unit; 311 acollimating lens; 312 a cylindrical lens; and 313 a polygon mirror. Inthe embodiment, a hexahedron mirror is used as a polygon mirror.Reference numeral 314 denotes a scanner motor to rotate the polygonmirror 313; 315 an image forming lens; 320 a BD mirror; 331 a photosensitive drum; 332 a transfer roller; 333 a charging roller; 334 adeveloping cylinder; and 335 a reflecting mirror.

In the controller unit 100, a reference numeral 101 denotes a maincontrol unit; 102 a scanner clock frequency divider to control arotational speed of the scanner motor 314 in correspondence to aresolution selection signal; 103 a BD counter; 104 an oscillator; 105 anamplifier serving as a laser driver for amplifying an APC control signaland a video signal and for emitting a laser beam; 106 an amplifier toamplify a PD signal in association with the laser emission; and 107 awaveform shaper to shape a waveform of the BD signal.

FIG. 8 shows the details of the relation between the controller unit 100and a scanner unit 340 in FIG. 7. In FIG. 8, the parts and componentssimilar to those in FIG. 7 are designated by the same reference numeralsand their detailed descriptions are omitted.

In the diagram, reference numeral 340 denotes the scanner unit having ascanner driver and control unit 341.

The scanner clock divider 102 of the controller unit 100 frequencydivides a reference signal from the oscillator 104 comprising a crystaloscillator in accordance with a divider ratio selection signal from themain control unit 101, thereby forming a scan clock signal andoutputting to the scanner unit 340. In the embodiment, the divider ratioselection signal is constructed by two bits and a total of four kinds ofrotational speed changeover signals can be output. However, therotational speeds which can be switched S are not limited to the aboveexample. It will be obviously understood that a further larger number ofbits are assigned to the divider ratio selection signal, therebyenabling an arbitrary kind of rotational speed to be selected and makingit possible to record and output at a number of kinds of resolutions.

In the scanner driver control unit 341, the scanner motor 314 is rotatedat a rotational speed corresponding to a scanner clock signal from thescanner clock divider 102. The scanner clock signal is compared with atach signal which is produced due to the rotation of the scanner motor314 as a reference signal of a phase locked loop (PLL) for the scannerclock signal. The rotation of the scanner motor 314 is controlled sothat both of the scanner clock signal and the tach signal alwayscoincide. Thus, the constant speed control of the scanner motor 314corresponding to a desired resolution can be executed.

The operation control of the embodiment having the above constructionwill now be described hereinbelow. The control timings of the embodimentwhich will be explained hereinafter are shown in a timing chart of FIG.9.

Since the recording apparatus has a construction such that therotational speed of the scanner motor 314 can be changed in accordancewith a desired resolution, the resolution (recording density) can bearbitrarily set. For instance, the resolution changeover such as 240 dpi⃡ 300 dpi, 300 dpi ⃡ 400 dpi, or the like can be executed by the sameapparatus.

During the rotation of the scanner motor 314, if a new resolution is setand input by an external control unit, an operation panel, or the like(not shown), the main control unit 101 outputs a correspondingresolution changeover signal according to the set input to the scannerclock divider 102. Due to this, the scanner clock divider 102 changesthe scanner clock signal and the rotational speed of the scanner motor314 is changed. In addition to it, a scanner ready signal indicatingthat the scanner motor 314 is rotating at a predetermined constantrotational speed is set to be false (not ready). Further, at this time,the laser output is inhibited and the rotational speed of the scannermotor 314 is also being changed, so that no BD signal is detected andthe BD not-ready state is set. The whole LBP apparatus of the embodimentis set into the not-ready state and the printer ready signal is set tobe false. At the same time, an input video signal from the externalcontrol unit or the like is masked by an AND circuit (not shown) in theBD counter 103 and a laser diode enable signal is produced.

On the other hand, at this time, since the image formation before theresolution is changed has been completed, a development/transfer outputis set to the off state. However, the developing/transfer system of theembodiment uses an image forming process by the roller charging/transferoperation. Particularly, a voltage within range about from--hundreds ofV to -1 kV (in the embodiment, -1 kV) is applied as a transfer output inorder to clean the transfer roller. Therefore, even if thedevelopment/transfer output is in the off state, the toner depositedonto the transfer roller can be transferred onto the photo sensitivedrum 331.

The scanner driver control unit 341 changes the rotational speed of thescanner motor 314 in correspondence to the scanner clock signal, therebycontrolling the rotational speed to a predetermined rotational speedwhich is specified by the resolution changeover signal (divider ratioselection signal). When the scanner motor has reached a predeterminedconstant speed state, a scanner ready signal is set to "1" (ready).

Upon reception of the scanner ready signal of the "1" level, the maincontrol unit 101 executes the following control to the transfer outputat least after the development output was turned off (after the offstate was confirmed).

In a manner similar to the foregoing transfer output, the cleaningsequence using the voltage of about--hundreds of V to -1 kV iscontinued.

Subsequently, a predetermined positive side bias voltage smaller than anordinary transfer output is applied. Particularly, this method is aneffective way to erase a trace of the recording medium and is used toequalize impedances between the photo sensitive drum 331 and thetransfer roller 332 in the cases where the recording medium exists andwhere it does not exist. Such a bias voltage V_(t0) is determined by amechanism and a construction of the LBP. In the embodiment, V_(t0) isset to about 2 kV which is equal to the half voltage of the ordinarytransfer output. However, such a voltage value is not limited to theabove value but depends on the environmental conditions, particularly,the humidity or characteristics of the recording medium. It is desirableto set the optimum voltage value by using those condition values asparameters. For example, it is also possible to automatically adjust thevoltage value to a desired predetermined transfer voltage value on thebasis of the humidity information. On the other hand, with respect tothe recording medium, particularly, a using style of special papers bythe hand-insertion paper feeding mode is also considered. In this case,by transmitting recording medium information through the operationpanel, host computer, or the like, the voltage value can be alsoautomatically adjusted in a manner similar to that mentioned above.

After the transfer output was set, the laser diode enable output is madeactive.

Then, in order to obtain the BD signal serving as a horizontal syncsignal, the amplifier 105 is energized and the laser is turned on. Thus,the laser beam is led to the BD detector through an optical fiber by theBD mirror 320. The waveform of the laser beam is shaped by the waveshaper 107. After that, the laser beam is sent to the BD counter 103 andthe BD signal is detected. After the BD signal was detected, theexposure is executed until the pulses of the number corresponding to atleast two lines (two main scanning operations) are counted in order todiscriminate whether the BD period is correct or not. Thus, the laserbeam is irradiated onto the photo sensitive drum 331 (into the imagearea).

However, as mentioned above, since the development output is set to theoff state at this time point, no latent image is produced on the photosensitive drum 331. In the embodiment, the sequence to clean thetransfer output is further executed, thereby making it difficult totransfer the toner to the transfer roller.

Consequently, the clean state can be held for a long time.

In FIG. 9, t denotes a time when the BD sync signal becomes effectiveafter the laser beam was irradiated onto the drum 331 and a timecorresponding to at least one or more rotations of the transfer roller332 after the latent image formed within the above time has passedthrough a transfer contact with the transfer roller 332.

FIG. 10 shows the relations among the time when the BD sync signalbecame effective after the laser beam had been irradiated to the photosensitive drum 331 in the cleaning sequence of the transfer output andthe respective sections at the time t until the transfer roller 332rotates at least one or more number of times after the latent imageproduced within the above time passed through the transfer contact withthe transfer roller 332. In the diagram, V indicates a peripheralvelocity of the photo sensitive drum 331 (or transfer roller 332).

After the time t elapsed after the BD signal had been made ready, theprinter ready signal is set to "1", thereby setting the printer to theready state.

After that, the ordinary print sequence is again started by a printsignal from the outside.

The above description has been made for the transfer output in the casewhere the print signal is immediately output after completion of theprinter ready operation. If the print signal is not output even afterthe elapse of a predetermined time, the transfer output is biased to anegative potential and the cleaning sequence is started.

As described above, according to the fourth embodiment, when therotational speed of the scanner motor 314 is switched, the formation ofthe unnecessary electrostatic latent image which is caused when thelaser beam was continuously emitted to obtain the BD signal as ahorizontal sync signal can be effectively prevented.

(Fifth embodiment)

Although the above description has been made with respect to an examplein which the developing/transfer system uses the image forming processby the roller charging/transfer operation, the invention is not limitedto the above example but can be also applied to the electrophotographicprocess in which the developing/transfer system is charged by a chargerconstruction.

FIG. 11 shows a constructional diagram of an LBP in the fifth embodimentaccording to the invention in the case where the developing/transfersystem was realized by the electrophotographic process by the chargingby the charger construction.

In the fifth embodiment, a construction other than thedeveloping/transfer system is similar to the fourth embodiment and thedetailed description of the similar construction is omitted.

A cartridge 350 shown in FIG. 11 is a unit which can be integratedlyexchanged. In the cartridge unit 350, reference numeral 351 denotes ablade; 352 indicates a primary charger; 353 a pre-exposing lamp; and 354a cleaner blade. The cartridge unit 350 also has the photo sensitivedrum 331 and the developing cylinder 334.

A transfer charger 356 and a deelectrifying needle 357 are also arrangedbelow the photo sensitive drum 331 of the cartridge 350. Anelectrostatic latent image formed on the drum 331 is copy transferredonto a recording paper which was conveyed by being synchronized byresist rollers 363.

Further, reference numeral 361 denotes a lower stage paper feed rollerand 362 indicates an upper stage paper feed roller. The rollers 361 and362 are used to feed the recording papers set in a lower or upper stagecassette (not shown) one by one. Reference numeral 363 indicates theresist rollers to adjust the conveying timing of the recording paperwhich was fed; 371 indicates a conveying guide portion to convey therecording paper on which the electrostatic latent image was transferredto a position between an upper fixing roller 372 and a lower fixingroller 373; and 374 represents a cleaner of a fixing device.

When the developing/transfer system as mentioned above is constructed bythe electrophotographic process by the charging by a chargerconstruction, the printer can be immediately made ready at the time ofthe BD ready after completion of the conversion of the rotational speedof the scanner motor 314. The printer ready signal can be soon set to"1".

Even in this case, when the rotational speed of the scanner motor 314 ischanged, at least the developing output is set to the off state in amanner similar to the fourth embodiment.

The transfer output is also set to the off state. Even in this case, forthe purpose of a countermeasure for the remaining paper trace, it isalso possible to apply the transfer output of V_(t1) to the transfercharger 356 so as to be equalized with the impedance at the time whenthe recording paper exists in the portion of the transfer charger 356.

Further, the transfer output V_(t1) can be also considered as a sum ofenergies. The voltage V_(t1) can be also pulse width modulated andoutput.

FIG. 12 shows a cleaning timing of the portion of the transfer charger356 in the fifth embodiment and subsequent control timings.

Even in the fifth embodiment, in a manner similar to the fourthembodiment, as shown in FIG. 12, when the rotational speed of thescanner motor 314 is switched, the formation of the unnecessaryelectrostatic latent image which is caused when the laser beam wascontinuously emitted to obtain the BD signal as a horizontal sync signalcan be effectively prevented.

As described above, according to the fourth and fifth embodimentsmentioned above, the copy transfer of the unnecessary electrostaticlatent image after the resolution was switched in the LBP can beprevented and the following effects are obtained.

(1) A buildup of dirt by the toner of the transfer roller 332 in theroller transfer system can be prevented.

(2) A dirt by the toner of the transfer charger 356 can be prevented.

Thus, a "back surface dirt" of the paper is eliminated and an image of ahigh quality without any dirt can be also provided even in theboth-sided printing mode.

(Sixth embodiment)

FIG. 13 is a block diagram showing an electrical construction of a laserprinter 94 including a printer engine unit 95 as a printing apparatusaccording to the sixth embodiment of the invention. FIG. 14 is a crosssectional view simply showing a construction regarding the printingoperation of the laser printer 94. Explanation will now be madehereinbelow with reference to FIGS. 13 and 14. The laser printer 94 has,for instance, a video controller unit 96. The print data which is outputfrom an external computer or the like is converted into an image signalby the video controller unit 96.

Reference numeral 51 denotes a laser element such as a laser diode orthe like. The laser element 51 is turned on and off on the basis of animage signal VDO which is given from the video controller unit 96 to thelaser driver 41 through a line 82. At this time, the video controllerunit 96 switches an output speed of the image signal VDO and outputs onthe basis of the set resolution.

On the other hand, various command signals including the designation ofthe resolution, change thereof, and the like are input from the videocontroller unit 96 to a microprocessor (hereinafter, referred to as an"MPU") 40 as control means through a line 83 and an interface circuit93. On the other hand, internal status information in the printer engineunit 95, error message which will be explained hereinlater, and the likeare output to the video controller unit 96 via an interface circuit 87and a line 88.

A laser beam from the laser element 51 passes through a collimating lensand a cylindrical lens (not shown) and is reflected to a rotary polygonmirror 52. A scanner driver 43 controls a scanner motor 53 on the basisof a signal from the MPU 40, so that the rotary polygon mirror 52 isrotated at a rotational speed corresponding to the set resolution. Alaser beam 80 which was reflected by the mirror 52 is reflected by areflecting mirror 54 and scans on a photo sensitive drum 71. A BD signalprocessing circuit 45 outputs a BD signal to specify a writing timing ofan image in the main scanning direction to the MPU 40 on the basis of adetection signal from a laser photo detector 55 arranged at the scanstart position of the laser beam 80.

At this time, the photo sensitive drum 71 is driven at a predeterminedrotational speed by a drum motor 56 and a drum driver 46 irrespective ofthe set resolution. The surface of the drum 71 has uniformly beencharged by a corona discharge of a primary charger 73. Therefore, anelectrostatic latent image is formed by scanning and exposing the drum71 by the laser beam 80. The electrostatic latent image is developed toa toner image by a developing unit 74 and is copy transferred onto arecording paper 61 as will be explained hereinlater. Reference numeral72 denotes a cleaner to clean the photo sensitive drum 71.

A construction to convey the recording paper 61 will now be described.Reference numeral 62 denotes a cassette to enclose the recording papers61. A plurality of kinds of cassettes 62 are prepared in accordance withthe sizes of recording papers 61 to be enclosed. One or a plurality ofprojection portions 62a are formed at a predetermined position(s) in thedirection perpendicular to the paper surface in the right edge portionof FIG. 14 of the cassette 62 in accordance with the size of recordingpaper 61 to be enclosed. When the cassettes 62 are attached to the laserprinter main body, the projecting portions 62a selectively push aplurality of micro switches 78 provided for the laser printer 94. Due tothis, the laser printer 94 recognizes the sizes of recording papers 61.That is, detecting means for detecting the sizes of recording papers 61is constructed by including the projecting portions 62a and microswitches 78.

Reference numeral 63 denotes a paper feed cam to feed the recordingpapers 61 enclosed in the arbitrary one of the cassettes 62 one by one.The recording paper 61 is conveyed to the position of paper feed rollers64a and 64b by the paper feed cam 63. Reference numeral 65 denotes aresist shutter to stop the recording paper 61 which is conveyed by therotations of the rollers 64a and 64b. At this time, the paper feedrollers 64a and 64b continue the rotations while slipping for therecording paper 61. When a resist solenoid 66 is driven synchronouslywith the timing of the image formation onto the photo sensitive drum 71and the resist shutter 65 is released upward in FIG. 2, the recordingpaper 61 is conveyed to the drum 71 by the paper feed rollers 64a and64b and conveying rollers 67a and 67b.

Reference numeral 75 denotes a transfer charger which is driven by ahigh voltage driver 85 on the basis of a signal from the MPU 40 and thetoner image on the drum 71 is transferred onto the recording paper 61 asmentioned above. Reference numerals 68a and 68b denote fixing rollers tothermally fix a toner image on the recording paper 61. Reference numeral84 denotes a heater to raise the temperature of the surface of thefixing roller 68a to a predetermined temperature. The recording paper 61printed as mentioned above is delivered onto a stacker 70 by deliveryrollers 69a and 69b.

The characteristic operation of the laser printer 94 with the aboveconstruction will now be described with reference to flowcharts of FIGS.15 and 16.

When a power source of the laser printer 94 is turned on, step T10 inFIG. 15 is first executed and the initialization processings such as toinitialize the content of a random access memory provided in the MPU 40and the like and to rotate the rotary polygon mirror 52 and the like areexecuted. At this time, the size of recording paper 61 is recognized bythe signals from the micro switches 78. In step T20, a communicationprocessing of a printer control command and the like are executed withthe video controller unit 96. That is, as will be explained hereinlater,a command communication control routine is executed and when a commandis received, interpretation and return processings of the command or aprocessing corresponding to the command received is executed. In themain routine in step T30, the well-known control of the printer and thelike are executed.

A changing operation of the resolution in the command communicationcontrol routine shown in step T20 will now be described with referenceto FIG. 16. First, in step T21, a check is made to see if a printercontrol command has been sent from the video controller unit 96 or not.If NO, the processing routine is returned. When the printer controlcommand is received, a check is made in step T22 to see if the receivedcommand is a resolution changeover request command or not. If a commandother than the resolution changeover request command has been received,in step T23, the MPU 40 interprets various kinds of received commandsand executes processings corresponding to the commands such as to drivea group of various devices such as primary charger 73 laser element 51,and the like through device drivers and the like.

For instance, when a print command is input, the drum motor 56 is driventhrough the drum driver 46 and the primary charger 73 is also madeoperative.

On the other hand, if a print density changeover request command hasbeen received in step T22, a check is made in step T24 to see if thetransfer charger 75 has been turned on or not. If YES, step T28 followsand error information is transmitted from the printer engine unit 95 tothe video controller unit 96. The error information is output as one ofstatus information for, e.g., the resolution changeover request command.Due to this, the video controller unit 96 can recognize the fact thatthe switching of the print density in the printer engine unit 95 was notexecuted.

Upon reception of the error information, the video controller unit 96informs the fact to an external computer or the like or again transmitsthe resolution changeover request command, for instance, after thetransfer charger 75 was turned off, thereby switching the resolution forthe next and subsequent pages. Or, even in the case of continuouslyprinting a plurality of pages, the printing operation of the next andsubsequent pages is temporarily stopped and a proper processing can beperformed. On the other hand, in a construction such that the videocontroller unit 96 is omitted and the processings which are executed bythe video controller unit 96 are performed by an external apparatus, theabove proper processing is executed on the basis of a software providedin the external apparatus such as a computer or the like.

On the other hand, if the transfer charger 75 is in the off state instep T24, step T25 follows and a signal indicative of the resolutionfrom the video controller unit 96 is received. In the case where asetting value of the print density has been stored in the MPU 40 and theresolution is switched, the content is updated to the received settingvalue.

After a calculation processing, which will be explained hereinlater, wassubsequently executed, in step T26, a signal is output to the scannerdriver 43 and the polygon mirror 52 is rotated at a rotational speedcorresponding to the set resolution. Thus, the resolution in the subscanning direction is changed.

For instance, it is now assumed that a rotational speed of the rotarypolygon mirror 52 at a resolution of 240 dpi is set to R_(A), afrequency of video clocks is set to F_(A), a rotational speed of themirror 52 at a resolution of 300 dpi is set to R_(B), and a frequency ofvideo clocks is set to F_(B). In this case, there are the followingrelations. ##EQU1## In the case of changing the resolution, forinstance, from 240 dpi to 300 dpi by the resolution changeover requestcommand, the above calculations are executed by the MPU 40 and therotational speed R_(B) is output to the scanner driver 43.

On the other hand, the frequency F_(B) of the video clocks of the imagesignal indicative of the resolution in the main scanning direction isdetermined by the video controller unit 96 so as to satisfy the aboveequations.

In step T27, in association with the change of the rotational speed ofthe polygon mirror 52, to change the output timing of the BD signal,data corresponding to the resolution is given to the BD signalprocessing circuit 45.

FIG. 17 is a timing chart for explaining the operation timings in theprinting operation. When a signal PRINT rises at time t₁ as a printcommand, the MPU 40 receives it and sets a signal DRMD to a high leveland makes the scanner motor 53 and drum motor 56 operative. After theelapse of a predetermined period W₁, the primary charger 73 is turnedon. When the rotation of the scanner motor 53 becomes stable, a signalSRDY is set to the high level. Upon completion of the print ready state,the printer engine unit 95 generates a VSYNC request signal. After theelapse of a predetermined time W₂ after the vertical sync signal VSYNChad been output from the video controller unit 96, an image signal VDOis output. Synchronously with the vertical sync signal VSYNC, the resistshutter 65 is released and the transfer charger 75 is activated and thelike. A release period of the resist shutter 65, an ON period W₄ of thetransfer charger 75, and the like are set in correspondence to the sizeof the recording paper 61 which was detected as mentioned above.

In the sixth embodiment, as in the above description of the operation,the MPU 40 outputs the error information without switching theresolution in response to the resolution. changeover request command inthe ON period W₄ of the transfer charger 75. Since the on/off control ofthe transfer charger 75 is executed by the MPU 40, the MPU 40 recognizesthe on/off state of the transfer charger 75. Therefore, the aboveoperation can be realized by merely changing the software withoutneeding to add a new construction. Moreover, since the period W₄corresponds to the size of recording paper 61, the ineffective periodcan be set to a proper length and is not set to a long time in vain.

As mentioned above, the sixth embodiment has been described with respectto the case where the period when the error information is outputwithout performing the resolution changeover operation in response tothe resolution changeover request command has been set to the period W₄when the transfer charger 75 is in the on state. However, in theinvention, the period to output the error information can be also setto, for instance, a period when the video signal of one page is beinginput or an on period of the primary charger 73 if it is a periodcorresponding to the size of recording paper 61. The above period can bealso set to other period such as a period corresponding to the length inthe conveying direction of the recording paper 61 after the conveyanceof the recording paper 61 was restarted after the resist shutter 65 hadbeen released.

On the other hand, in place of the resolution changeover requestcommand, only a resolution designation command is used and it is alsopossible to construct in a manner such that a newly designatedresolution and the present resolution are compared and only when theydiffer, the processing is executed similarly to the resolutionchangeover request command.

In addition, it is also possible to construct in a manner such that aresolution changeover permission flag is provided and error informationis output in response to the resolution changeover request command whichwas input when the flag is not set to "1".

Further, although the case of the laser printer in which the main scanis performed by using a rotary polygon mirror has been described, theinvention can be also similarly embodied even in the case of a printingapparatus such that a latent image is formed onto a photo sensitivematerial by using an LED array or a liquid crystal shutter or the like.

What is claimed is:
 1. An image forming apparatus comprising:a lightbeam generator which generates a light beam which is modulated on thebasis of image information; a rotary polygon mirror which scans thelight beam from the light beam generator on a photosensitive drum inorder to form an eletrostatic latent image on the photosensitive drum,said image forming apparatus performing an image forming operation atone of a plurality of rotational speeds of the rotary polygon mirror; adeveloping unit which forms a toner image by developing theelectrostatic latent image formed on the photosensitive drum; a transferunit which transfers the toner image formed on the photosensitive drumonto a recording medium; a light beam detecting element which detectsthe light beam scanned by said rotary polygon mirror in a non-latentimage forming region in order to obtain a horizontal sync signal;forcible light emission means for causing the light beam generator toemit the light beam, so that the light beam is detected by the lightbeam detecting element; and transfer unit charging means for chargingthe transfer unit to a second potential during a period when theforcible light emission means is causing the light beam generator toemit the light beam after the rotational speed of the rotary polygonmirror has been changed, wherein the second potential is different thana potential at which the transfer unit is charged when performing atransfer operation.
 2. An image forming apparatus according to claim 1,wherein the transfer unit includes a transfer roller.
 3. An imageforming apparatus according to claim 1, wherein the transfer unitincludes a transfer charger.
 4. An image forming apparatus according toclaim 1, wherein at least the photosensitive drum and the developingunit are enclosed into one cartridge.
 5. An image forming apparatusaccording to claim 1, wherein an operation of switching the rotationalspeed of the rotary polygon mirror is performed after a recordingoperation when a designation of changing the rotational speed of therotary polygon mirror is received during the recording operation.
 6. Animage forming apparatus according to claim 5, wherein the forcible lightemission means causes the light beam generator to emit the light beam inthe non-latent image forming region after the light beam was detected bythe light beam detecting element.
 7. An image forming apparatusaccording to claim 6, wherein the transfer unit includes a transferroller.